1. Field of the Invention
The present invention relates to stepping motors, and more particularly to stepping motors in small sizes whose stator outer diameter is 8 mm or less.
2. Related Background Art
In conventional small-sized stepping motors, a ratio (R) between the thickness (t) of pole teeth formed on a stator core and an outer diameter (D) of a rotor magnet is typically 0.8 through 0.12 (R=0.8 through 0.12). Many of the stepping motors on the market have the ratio R ranging between 0.14 and 0.119 (R=0.14 through 0.119).
Small-sized stepping motors that are used in small device, such as, for example, notebook type personal computers are in a generally oval shape whose external diameter is 8 mm or less (shorter width is 6.5 mm or less). Since small-sized stepping motors have tight spatial restrictions, they have been developed with an emphasis on improvements of their torque for the necessity to satisfy torque requirements.
It is important to secure a good torque characteristic in miniaturizing motors. However, if the emphasis is placed only on securing the torque characteristic in designing stepping motors, the linearity characteristic, which is a unique characteristic of stepping motors, may deteriorate.
In other words, the magnetomotive force of both of the rotor and stator may preferably be made greater in view of securing the torque characteristic. To improve the magnetomotive force of the stator, a coil volume of coils that are wound on a bobbin inside a core of the stator may be increased. To increase the coil volume, the thickness of pole teeth formed on the core may be reduced, which is believed to cause the aforementioned problem of deteriorated linearity characteristic.
The inventor of the present invention has conducted experiments and discovered that the linearity characteristic of stepping motors deteriorated because the magnetic paths formed by the pole teeth are saturated as a result of thinning the pole teeth.
The deterioration of the linearity characteristic that is caused by thinned pole teeth may not pose a major problem in large-sized stopping motors. However, only when stepping motors are miniaturized, the deterioration of the linearity characteristic becomes significant, and therefore such problems provide a peculiar blind spot in designing stepping motors.